1. Field of the Invention
The present invention relates to a cantilever type displacement element which is displaced by the converse piezoelectric effect. The present invention also relates to a cantilever type probe employing the above element, and a scanning tunneling microscope and an information processing apparatus employing the above probe.
2. Related Background Art
In recent years, a scanning tunneling microscope (hereinafter referred to as "STM") has been developed which enables direct observation of electronic structure of atoms on the surface of a conductor (G. Binnig et al. Phys. Rev. Lett. 49 (1982) 57). The STM has made practicable the measurement of a real space image of a single crystal material as well as of an amorphous material with exceedingly high resolution power (nanometer or finer).
The technique of STM is being studied in various application fields such as observation, evaluation, and micro-machining of semiconductors and high polymer materials at an atomic order or a molecular order (E. E. Ehrichs, 4th International Conference on Scanning Tunneling Microscopy/Spectroscopy, '89, S-13-3), and in application fields of recording-reproducing apparatuses.
For processing of calculation information by computers, etc., in particular, the recording apparatus is required increasingly to have a larger capacity. Furthermore, the recording apparatus is required to be miniaturized as the result of miniaturization of microprocessors and improvement of computation ability resulting from the progress of semiconductor processing technique. To satisfy such requirements, a recording-reproducing apparatus is disclosed (U.S. Pat. No. 4,575,822), which apparatus conducts recording by changing the work function of the surface of a recording medium by application of voltage by means of a transducer comprising a tunnel current-generating probe provided on a driving means which is capable of changing finely the distance from the recording medium, and conducts reading by detecting the change of the tunneling current caused by the change of the work function. This apparatus has minimum recording areas of 10 nm square.
In such an apparatus, the probe is required to scan a sample within a range of from several nm to several .mu.m, and therein a piezoelectric element is used as the moving mechanism. An example of the moving mechanism is a tripod type one in which three piezoelectric elements are arranged perpendicularly to each other in x-y-z axis directions and a probe is provided at the intersection point of the axes. Another example is a cylinder type one in which divided electrodes are provided on the peripheral surface of a cylindrical piezoelectric element which is fixed at one end and is provided with a probe at the other end, and the cylinder is deformed in accordance with the respective divided electrodes for scanning.
Recently, a probe-moving mechanism has come to be formed finely by micro-machining by utilizing semiconductor machining technique (K. E. Peterson, IEEE Trans. on Electron Devices, Vol. ED-25, No. 10, p. 1241, 1978). FIGS. 5A and 5B illustrate a cantilever composed of a piezoelectric bimorph formed on an Si substrate by micro-machining technique (T. R. Albrecht, "Microfabrication of Integrated Scanning Tunneling Microscope", Proceedings of 4th International Conference on Scanning Tunneling Microscopy/Spectroscopy, '89, S10-2).
FIG. 5A is a perspective view of the cantilever, and FIG. 5B is a cross-sectional view thereof at the line A--A' in FIG. 5A. The cantilever is formed on a substrate 1 by laminating a layer of bisected electrodes 2a, 2b, a piezoelectric ZnO layer 3, an intermediate electrode 4, a piezoelectric ZnO layer 5, and bisected electrodes 6a, 6b, and then removing a part of the Si substrate by anisotropic etching so that the cantilever is supported at its one end by the unremoved substrate portion. At the tip of the cantilever constructed of the piezoelectric bimorph, a probe 7 made of metal or the like is attached by adhesion or other bonding method to detect tunneling current through a drawing-out electrode 16. Such a cantilever, which has a bimorph construction, is capable of giving a large displacement especially in a vertical direction advantageously.
The micro-machining technique enables fine construction of the probe-driving mechanism, and makes easier the multiplication of the probe for raising the speed of writing and reading of information with the recording-reproducing apparatus. The probe-driving mechanism itself prepared by utilizing a thin film technique for a piezoelectric material can be incorporated advantageously into an IC process which is primarily used for Si semiconductors.
The material for the electrode of the cantilever, in most cases for device fabrication, is Al or Au which has a relatively low melting point and characteristically makes easier the thin film electrode formation.
In preparation of the element shown in FIGS. 5A and 5B, many layers of electrodes and piezoelectric substances have to be laminated. Therefor, the thickness and the internal stress of each layer is required to be strictly controlled because the cantilever which has a thin film lamination construction may cause peeling at the interfaces between the electrode and the piezoelectric material owing to the stress in the layers.
The piezoelectric film is composed of a nitride or an oxide. At the interface between the piezoelectric material and the electrode, materials of completely different natures are bonded, which may cause great stress therein. In the case of laminated thin films, the entire stress at the interfaces is not negligible in comparison with the case of a bulky or thick material. Especially, a large difference of the internal stresses between the electrode film and the piezoelectric film will give rise to many problems. Therefore, smaller internal stresses of the respective layers and a smaller difference of the stresses are desired.
When a base metal such as Al is used as the electrode material, a passivation film is desirably provided to prevent corrosion. However, the lamination of a passivation film on a thin film cantilever makes the aforementioned control of the stress more difficult, and the passivation film may be cracked by the bending motion of the element itself. Accordingly, a high level of technique is required for making an element of high durability.
When Au, a noble metal, is used as the electrode material, the electrode tends to have a large remaining internal stress, although the electrode is chemically stable and less reactive to the piezoelectric material and has excellent properties.
FIG. 3 shows the dependence of the internal stresses of thin metal films on heat treatment temperature. The heat treatment was conducted at 200.degree. C., the temperature of the substrate required for obtaining satisfactory crystalline thin film of the piezoelectric material like zinc oxide. As shown in FIG. 3, Au has relatively small internal stress immediately after the film formation, whereas it comes to have a larger tensile stress after heat treatment at 200.degree. C. Consequently, the gold electrode after the formation of piezoelectric film has a large remaining internal stress, and tends to cause peeling of the film.